harsh or soft?
In an informal occasion, for example if asked to enter a restaurant or bar by an employee or if a waiter is asking whether you would want another drink or dessert, the most simple and straightforward way would be to say, "kekkoudesu" (meaning "no, it's okay") would be okay. This does not sound as harsh as saying "no" or "I don't want to."
If trying to say it in a softer way, you may want to add "sumimasen ga kyou wa kekkoudesu" (I'm sorry but I cannot go today. If trying to sound harsh, you may want to say "iyadesu" and walk away.